1. FIELD OF THE INVENTION:
The invention is directed to a switched-mode DC- to -DC which combines a flyback-type converter with a flux-type converter, containing at least a first and a second flyback transformer each with at least one primary winding and one secondary winding, wherein a timed primary switch is wired in series with the primary winding and one secondary switch is wired in series with each secondary winding.
2. DESCRIPTION OF THE RELATED ART
DC/DC power converters for converting a primary DC voltage into a secondary DC voltage are extensively known and in general use. Basically, one distinguishes between two types, the forward-type converter and the flyback-type converter. Flyback-type converters are only known as single phase circuits. Forward-type converters are built as single phase circuits and a push-pull type circuits.
The known flyback-type converters have a series of disadvantages. The calculation of the transformer requires lengthy repetitive processes, protective measures against transformer saturation, the effects of leakage inductance and exceeding of the breakdown voltage of the power switch must be taken, which greatly increases the quantity of the required components. The transformer transmits the energy only during half of each period, which tends to lower the overall efficiency of the circuit.
An increased energy flow and thus an improved efficiency result if push-pull circuits are used. Push-pull circuits are only known forward-type converters. Here, two power switches operate in push-pull action by means of two primary windings upon one transformer. Push-pull circuits require, however, very good balancing in the primary circuit, so that no DC current magnetic biasing arises in the transformer. In addition, specific protective measures must be taken in order to prevent the power switches from being simultaneously on line which would result in a short circuit.
A single-phase converter is known from the DE-pbulication "Elektronikpraxis", Oct. 1986, page 52 and following, which operates with two transformers, whose primary windings are wired in series and whose secondary windings are wired in parallel through each one rectifier. Through an appropriate polarity of the primary and secondary windings and the rectifiers, the one transformer works as forward-type while, the other transformer works as a flyback-type converter. This type of circuit has certain advantages compared to the basic circuits, for instance, no protective measures are required against exceeding the breakdown voltage of the power switch. In spite of that the quantity of components is still high because two transformers have to be used. Also, this circuit can only be operated as a single-phase converter. This means that each transformer transmits power only during one-half of each period.
A refinement of this circuit is known from the Japanese patent application No. 60-170461(A). Two transistors actuated in push-pull action work upon four transformers, of which, respectively, two operate as single phase flyback-type converters and the other two as single-phase forward-type converters with the help of rectifier diodes switched at the secondary windings for generation of secondary voltage. Here, also, each transformer transmits power only during half of each period.
A single-phase DC/DC power converter with a transformer and one primary winding is known from the U.S. Pat. No. 4,455,596. The transformer has two secondary windings, one of which acts as a flyback-type converter with the help of a rectifier diode of suitable polarity, and the other acts as a forward-type converter with the help of a rectifier diode of suitable polarity. The transformer transmits energy during both halves of each period. Of course, the energy transmitted during the flux phase must be temporarily stored in a memory choke and can only be released to a secondary circuit during the subsequent blocking phase. The necessity to utilize a memory choke and a second rectifier diode cancels the saving involved in utilizing a single transformer. Apart from that, both secondary voltages fluctuate greatly as a function of the respective load resistance, as this is known in flyback-type converters.